Control circuitry using a pull-down transistor for high voltage field terminated diode solid-state switches

ABSTRACT

To switch a first gated diode switch (GDS) to the &#34;OFF&#34; state requires a voltage applied to the gate which is more positive than that of the anode or cathode and the sourcing of current into the gate which is of the same order of magnitude as flows between the anode and cathode. Control circuitry, which uses a second GDS coupled by the cathode to the gate of the first GDS, is used to control the state of the first GDS. The state of the second GDS is controlled by a branch circuit having a relatively modest current handling capability. An n-p-n junction transistor has the emitter and collector coupled to the cathode and gate, respectively, of the first GDS, and has the base coupled through a p-n-p transistor to the input terminal of the control circuitry. The n-p-n transistor facilitates a quick turn-on of the first GDS by rapidly bringing the potentials of the gate and cathode of the first GDS to levels which are close together.

TECHNICAL FIELD

This invention relates to control circuitry for controlling the state ofsolid-state switches and, in particular, to control circuitry forcontrolling the state of solid-state switches which have high voltageand relatively high current capabilities.

BACKGROUND OF THE INVENTION

High voltage and relatively high current capability solid-stateswitches, such as one described in an article entitled "A FieldTerminated Diode" by Douglas E. Houston et al, published in IEEETransactions on Electron Devices, Vol. ED-23, No. 8, August, 1976, andthose discussed in pending U.S. patent applications Ser. No. 972,056,filed Dec. 20, 1978, and Ser. Nos. 972,021, 972,022, and 971,886, allfiled Dec. 20, 1978 (now abondoned), and having a common assignee withthe present application and U.S. patent applications Ser. Nos. 107,774,107,773, 107,772, 107,780, and 107,775, which were all filed Dec. 28,1979 with the present application and have a common assignee with thepresent application, have an ON (conducting) state and an OFF (blocking)state. These switches are capable of blocking relatively large potentialdifferences in the OFF state. Each of these switches has two outputterminals which are generally denoted as the anode and cathode, acontrol terminal which is generally denoted as the gate, and asemiconductor body whose bulk separates the anode, cathode, and gateregions. The parameters of the various portions of the semiconductor aresuch that with the potential of the anode region being greater than thatof the cathode region and the potential of the gate region beinginsufficient to cause the potential of a vertical cross-sectionalportion of the bulk of the semiconductor body between the anode andcathode to be greater in potential than the anode or cathode regionsthere is facilitated a substantial current flow between the anode andcathode regions via the bulk. With the potential of the gate regionbeing sufficiently more positive than that of the anode and cathoderegions to cause a vertical cross-sectional portion of the bulk of thesemiconductor body between the anode and cathode regions to be morepositive in potential than the anode and cathode regions there isfacilitated an interrupting or inhibiting of current flow between theanode and cathode regions. The magnitude of the needed gate potentialnecessary to turn off these switches is a function of the geometry anddoping levels of the semiconductor regions of each switch and of theanode and cathode potentials.

Control circuitry used to apply a blocking voltage to the gate terminalof each of these switches must be able to sustain a more positivevoltage than is at the anode and cathode terminals and must be able tosupply current which is generally of the same magnitude as flows throughthe anode and cathode of each switch.

U.S. patent applications Ser. Nos. 972,023 and 972,024, filed Dec. 20,1978 (now abondoned), and have a common assignee with the presentapplication, describe and illustrate control circuitry which itself usesa high voltage and current switch of the type described hereinabove tocontrol the state of a similar switch. If a control circuit should failto break (interrupt) current flow through on ON switch connectedthereto, it is necessary to electrically disconnect the controlcircuitry from one of the supply potential sources. The controlcircuitry is then reset and reconnected to the potential source. It isthen activated again so as to break conduction through the ON switch.

Usually a conventional high voltage and high current capability switchis used between the high voltage source and the control circuitry. Thisswitch can be an optically activated switch. Generally it is arelatively expensive component and only one is used for a relativelylarge number of control circuits. If any of the switches to becontrolled fails to turn off, it is necessary to disconnect all thecontrol circuits from the power supply. This may result in all of theswitches connected to the control circuitries being switched to the ONstate independent of which state is desired. This is undesirable in someswitching applications. The speed of operation and power dissipation ofthe above-described control circuitry may be slower and higher,respectively, than is desired in some switching applications.

It is desirable to have circuitry capable of controlling high voltageand high current solid-state switches of the type discussed hereinabovewhich has improved switching time and lower power dissipation than priorart circuitry, and which can maintain some switches connected thereto inthe desired state even if one of the switches being controlled fails tobreak current (assume the OFF state).

Copending U.S. patent application Ser. No. 107,777, filed Dec. 28, 1979with the present application and in which there is a common assigneewith the present application, contains subject matter similar to that ofthe present application.

SUMMARY OF THE INVENTION

The present invention is directed to control circuitry for controllingthe state of high voltage and relatively high current solid-stateswitches of the type described hereinabove. This circuitry essentiallycomprises a control switch (GDSC) which, in a preferred embodiment, is agated diode switch, a branch circuit coupled thereto a control the statethereof, and a circuit means which in the preferred embodiment is ann-p-n transistor (T1) whose emitter is coupled to one of the outputterminals of a load switch (GDSL1) which is to be controlled and whosecollector is coupled to the gate of GDSL1. The base of T1 is coupled tothe branch circuit which, in turn, is coupled to an input terminal ofthe control circuitry. One output terminal of GDSC, typically, thecathode terminal, is coupled to the gate of GDSL1.

With a low, a "0", input signal applied to the input terminal, T1becomes biased ON and conducts such that the potential of the collectorand emitter become fairly close to each other. These are also the gateand cathode or anode terminals of GDSL1. Accordingly, GDSL1 is switchedto an "ON" (conducting) state if proper operating potentials are appliedto the anode and cathode terminals. The branch circuit causes GDSC to beturned to the OFF state when it is desired that GDSL1 be in the ONstate. When it is desired to switch GDSL1 to the OFF (nonconducting)state the potential of the input terminal is brought to a high, a "1",level. This causes GDSC to be switched to the ON state, and thus couplesthe potential of a potential source coupled to its anode to the gate ofGDSL1. In addition, positive charge flows from the potential source intothe gate of GDSL1. There is provided a sufficiently positive potentialon the gate of GDSL1 and a sufficient current flow into the gate toswitch GDSL1 to the OFF state.

The circuit means comprising n-p-n transistor T1 quickly discharges thepotential of the gate of GDSL1 to a level at which the ON state can beachieved. This results in GDSL1 being able to rapidly switch to the ONstate soon after the input signal is pulsed to the low level.

These and other features and advantages of the invention are betterunderstood from a consideration of the following detailed descriptiontaken in conjunction with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 illustrates a solid-state bidirectional switch;

FIG. 2 illustrates a solid-state structure;

FIG. 3 illustrates control circuitry in accordance with one embodimentof the present invention; and

FIG. 4 illustrates control circuitry in accordance with anotherembodiment of the present invention.

DETAILED DESCRIPTION

Referring now to FIG. 1, there is illustrated a semiconductor structure10 comprising two gated diode switches, GDS1 and GDS2, which areillustrated within dashed line rectangles and are both formed on acommon support member 12. Structure 10 is disclosed and described in acopending U.S. patent application Ser. No. 107,776, filed Dec. 28, 1979with this application and in which there is a common assignee. GDS2 isdisclosed and described in copening U.S. patent application Ser. No.107,774, which has a common assignee and was filed Dec. 28, 1979 withthis application. Support member 12 is typically a semiconductor waferor a substrate. Dielectric layers 14 and 14a separate monocrystallinesemiconductor bodies 16 and 16a, respectively, from support member 12and from each other. Support member 12 has a major surface 11 and bodies16 and 16a each have a portion that is common with surface 11. Whereasonly two gated diode switches are illustrated, a plurality ofdielectrically isolated gate diode switches of the type of GDS1 and/orGDS2 can be formed in a common semiconductor wafer or substrate 12.

GDS1 and GDS2 are illustrated having electrical connections therebetweenwhich facilitate the use thereof as a bidirectional high voltage switch.GDS1 and GDS2 need not be electrically connected and each can functionindependently of the other.

In one typical embodiment, support member 12 is a semiconductor wafer(substrate) of n or p type conductivity and semiconductor bodies 16 and16a have bulk portions thereof which are of p-type conductivity. Thesemiconductor regions contained within semiconductor body 16 are verysimilar to those contained in body 16a. A localized anode region 18,which is typically of p+ type conductivity, is included in body 16 andhas a portion thereof that extends to surface 11. Surrounding anoderegion 18 is a p type region 42 which also has a portion thereof whichextends to surface 11. Surrounding region 42 is a p- type region 43which also has a portion thereof which extends to surface 11. Theconductivity of region 42 is intermediate between that of anode region18 and semiconductor body 16. The conductivity of region 43 isintermediate between that of region 42 and semiconductor body 16.Electrode 28 is illustrated making contact to region 18. Electrode 28 isseparated from portions of surface 11 other than those over the exposedportion of region 18 by dielectric layer 26. A localized gate region 20of n+ type conductivity is included in body 16 and has a portion thereofwhich extends to surface 11 and is separated from region 42 by portionsof the bulk of semiconductor body 16. An electrode 30 contacts region 20at surface 11. Electrode 30 is separated from portions of surface 11other than those over the exposed portion of region 20 by dielectriclayer 26. A localize cathode region 24, which is of n+ typeconductivity, is included in body 16 and is separated from region 20 byportions of the bulk of semiconductor body 16. Region 24 is surroundedby a p+ type guard ring 40 which, in turn, is surrounded by a p typeregion 22 which, in turn, is surrounded by a guard ring like p typeregion 46. Region 46 can extend, as is illustrated by the dashed line,to essentially completely surround region 22, except for the portionsthereof common to surface 11. Region 46 is separated from regions 20 and43 by portions of the bulk of semiconductor body 16. Electrode 32contacts region 24 and a separate electrode 50 contacts region 40. Bothelectrodes are separated from portions of surface 11 other than over therespective exposed portion of regions 24 and 40. A layer 48 of n typeconductivity exists between the dielectric layer 14 and semiconductorbody 16. Layer 48, which is part of a preferred embodiment, is shown indashed line since it is optional. Gate region 20 also serves as thecollector of a lateral n-p-n transistor with cathode region 24 servingas the emitter and regions 46, 22, and 40 serving as the base.

Semiconductor body 16a contains regions which are very similar to thosecontained within semiconductor body 16 with the exception of the factthat the p+ type guard ring region 40a does not have a separateelectrical contact thereto as does guard ring 40.

GDS1 is typically operated as a switch which is characterized by a lowimpedance between anode region 18 and cathode region 24 when in the ON(conducting) state and as a high impedance between said two regions whenin the OFF (nonconducting) state. With operating potentials applied toanode region 18 and cathode region 24, the potential applied to gateregion 20 determines the state of the switch. Conduction between anoderegion 18 and cathode region 24 can occur if the potential of the gateregion 20 is near or below the potential of the anode region 18, cathoderegion 24, and region 22. During the ON state holes are injected intobody 16 from anode region 18 and electrons are injected into body 16from cathode region 24. This effectively lowers the resistance of body16 such that the resistance between anode region 18 and cathode region24 is relatively low when GDS1 is operating in the ON state. This typeof operation is denoted as dual carrier injection and the type ofstructure described therein has been denoted as a gated diode switch(GDS). Guard ring region 40 and regions 22 and 46 help limit thepunch-through of a depletion layer formed during operation between gateregion 20 and cathode region 24 and help to inhibit the formation of asurface inversion layer between these two regions. In addition, theyfacilitate gate region 20 and cathode region 24 being relatively closelyspaced apart. This facilitates relatively low resistance between anoderegion 18 and cathode region 22 during the ON state.

Substrate 12 is typically held at the most positive potential levelavailable. Conduction between anode region 18 and cathode region 24 isinhibited or cut off if the potential of gate region 20 is sufficientlymore positive than that of anode region 18 and cathode region 24. Theamount of excess positive potential needed to inhibit or cut offconduction is a function of the geometry and impurity concentration(doping) levels of GDS1. This positive gate potential causes a portionof body 16 between gate region 20 and the portion of dielectric layer 14therebelow to be at a potential that is more positive than that of anoderegion 18, cathode region 24, and region 22. This positive potentialbarrier inhibits the conduction of holes from anode region 18 to cathoderegion 24. It essentially pinches off body 16 against dielectric layer14 in the bulk portion of semiconductor body 16 below gate region 20 andextending down to dielectric layer 14. It also serves to collectelectrons emitted at cathode region 24 before they can reach anoderegion 18. Examples of control circuitry capable of supplying the neededgate potentials and absorbing the electrons are illustrated anddescribed in U.S. patent application Ser. Nos. 972,023 and 972,024, nowabandoned. Other control circuitry in accordance with the presentinvention for controlling gated diode switches like GDS1 and/or GDS2 isillustrated and described in FIGS. 3 and 4 herein.

The ON state can be achieved by allowing gate electrode 30 toelectrically float in potential, forward-biasing the anode region 18with respect to the cathode region 24, and applying a potential toelectrode 50 which forward-biases the emitter-base junction comprisingbase regions 46, 22, and 40 and emitter (cathode) region 24. Once GDS1is on, if electrode 50 is allowed to electrically float in potential,then GDS1 can be maintained in the ON state with the potential of gateregion 20 at the same or a more positive level than anode region 18 andcathode region 24, so long as the potential of gate region 20 is belowthe level which essentially completely depletes a verticalcross-sectional portion of semiconductor body 16 between anode region 18and cathode region 24 and from surface 11 to the top of dielectric layer14 and causes the potential of this cross-sectional portion to begreater than that of the anode, cathode, and regions 22, 40, 46.

As has been earlier denoted, region 20, in addition to serving as thegate terminal of the gated diode switch, serves as the collector of alateral n-p-n transistor with regions 46 and 22 serving as the base andregion 24 serving as the emitter. Region 40, which also serves as partof the base, is typically of p+ type conductivity and, thus, serves asan electrical contact to region 22. Electrode 50 contacts region 40 andthus allows the base of the n-p-n transistor to be controlled. Ifelectrode 50 is held at a positive potential with respect to electrode32, then the n-p-n transistor is biased on and the potential betweengate region 20 (the collector of the transistor) and cathode region 24(the emitter of the transistor) is relatively small. Typically thiscollector-emitter voltage is on the order of several tenths to severalvolts. The potential of gate region 20 is thus drawn to a level close tothat of the cathode regions 24. This effectively reduces the gatepotential to a level which is insufficient to cause the GDS to be in theOFF state with proper operating potentials applied to the anode andcathode regions. The adjusting of the potential applied to electrode 50to forward-bias the n-p-n transistor facilitates GDS1 being relativelyrapidly switched to the ON state.

Semiconductor body 16a contains essentially the same regions assemiconductor body 16, except that guard ring 40a does not have anelectrical contact thereto. Thus, no external control of the basepotential of the n-p-n transistor which comprises region 20a as thecollector, regions 46a, 22a, and 40a as the base, and region 24a as thecathode is possible. The size of GDS1 is only slightly larger than thatof GDS2. The portion of region 40 which is contacted by electrode 50 issomewhat larger than the corresponding region 40a and, accordingly,region 22 is somewhat larger than regin 22a.

The electrical connections shown between electrodes 28 and 32a to aterminal X, electrodes 30 and 30a to a terminal G, and electrodes 32 and28a to terminal Y, couple GDS1 and GDS2 together so as to form abidirectional switching element whose equivalent circuit is illustratedin FIG. 3 herein.

Referring now to FIG. 2, there is illustrated a structure 10,000comprising a semiconductor support member 12,000 having a major surface11,000 and a monocrystalline semiconductor body 16,000 whose bulk is ofp type conductivity and is separated from support member 12,000 by adielectric layer 14,000. A p+ type conductivity contact region 34,000and an n+ type conductivity region 35,000 exist within a portion ofsupport member 12,000 and are both coupled to an pelectrode 36,000coupled thereto. Electrode 36,000 is electrically isolated from allportions of surface 11,000, except where it contacts regions 34,000 and35,000, by a dielectric layer 26,000.

Support member 12,000 could, and usually does, include at least oneother dielectrically isolated semiconductor bodies (not illustrated)like semiconductor body 16 of FIG. 1. Localized first and secondseparated p+ type conductivity regions 18,000 and 24,000 are included inbody 16,000 with each having a portion that forms a part of surface11,000. A localized region 42,000 of p type conductivity encirclesregion 18,000. A localized region 22,000 of p type conductivityencircles region 22,000 and is itself encircled by a region 46,000 whichis of p- type conductivity. Semiconductor regions of relatively lowresistivity are denoted as of p+ or n+ type conductivity. Those ofrelatively high resistivity are denoted as of p- or n- typeconductivity. Those of intermediate resistivity are denoted as p or ntype conductivity. A localized n+ type conductivity region 20,000 isincluded in body 16,000 and is located in between regions 18,000 and24,000. An electrode 28,000 is coupled to regions 18,000 and 20,000. Aseparate electrode 32,000 is coupled to region 24,000. Electrodes 28,000and 32,000 are separated from surface 11 except where they contact therespective regions by dielectric layer 26,000.

Structure 10,000 acts essentially as a pinch resistor with region 20,000pinching off semiconductor body 16,000 to create a relatively highresistance region between the bottom of region 20,000 and the top ofdielectric layer 14,000. Structure 10,000 acts to limit current flowbetween regions 18,000 and 24,000. Within a first range of potentialdifference between regions 18,000 and 24,000 the resistance between thetwo regions is essentially constant and the current increases linearlywith voltage. Once this range is exceeded, the electrical field createdunder electrode 28,000 tends to effectively further pinch off theportion of semiconductor body 16,000 under region 20,000. This increasesthe resistance between regions 18,000 and 24,000 and thus limits currentflow from one region to the other as voltage across the regionsincreases. Structure 10,000 thus acts as a resistor and as a currentlimiter.

Referring now to FIG. 3, there is illustrated a switching sytem 300comprising control circuitry 310 (within the largest dashed linerectangle) which is coupled by an output terminal 332 to the gateterminals of a pair of high voltage switching devices GDSL1 and GDSL2.The anode of GDSL1 and the cathode of GDSL2 are coupled to a firstterminal YO and to a resistor R6 and the cathode of GDSL1 and the anodeof GDSL2 are coupled to a second terminal XO and to a resistor R5. Thiscombination of GDSL1 and GDSL2 functions as a bidirectional switch whichselectively couples terminals XO and YO via a relatively low resistancepath through GDSL1 or GDSL2. For illustrative purposes, these switcheswill be assuned to comprise the gated diode switch structure illustratedin FIG. 1. Control circuitry 310 functions so as to supply the neededpotentials at terminal 332 and the current sourcing or sinkingcapability necessary to control the state of GDSL1 and GDSL2.

Control circuitry 310 essentially comprises a high voltage switch GDSC,a first voltage branch circuit 310A (illustrated within a dashed linerectangle) and a second voltage branch circuit 310B (illustrated withinanother dashed line rectangle). Branch circuit 310A maintains the loadswitches GDSL1 and GDSL2 in an ON state such that conduction can occurthrough one or the other load switch if the potential of the anode andcathode terminals thereof is sufficient to support conduction or it caninhibit conduction through the load switches by maintaining the loadswitches in an OFF state. Branch circuit 310B serves to help switchGDSL1 and GDSL2 to an OFF state and therefore helps interrupt or inhibitconduction between XO and YO independent of the potentials appliedthereto so long as these applied potentials are within preselectedlimits.

Control circuitry 310 comprises a high voltage switch GDSC, which forillustrative purposes is the switch structure GDS2 illustrated in FIG.1, a first current limiter CL1, which for illustrative purposes is thestructure illustrated in FIG. 2, n-p-n transistors T1, T6, T7, and T8,p-n-p transistors T2, T3, T4, and T5, n-p-n phototransistors T9 and T10,which each have a photosensitive base region 341, 345, a diode D1,resistors R1, R2, R3, and R4, and a capacitor C1. A first input terminal312 is coupled to R1, which is in turn coupled to the bases of T2 and T3and to a terminal 314. The emitter of T2 is coupled to a terminal 316and to R2, which is also coupled to a terminal 318, the emitter of T3and the base of T4. The collector T2 is coupled to the base of T1 and toa terminal 320. The collector and emitter of T1 are coupled to outputterminal 332 and terminal XO, respectively. The collector of T3 iscoupled to the collector of T6, the gate of GDSC, and to a terminal 322.The emitters of T4 and T5 are coupled together to a terminal 324 whichis coupled to a first voltage supply (source) V1. The collector of T4 iscoupled to the base of T5, one terminal of CL1, and to a terminal 326. Asecond terminal of CL1, in a preferred embodiment, is coupled to thecathode of GDSC and terminal 332, but it can instead be coupled to powersupply (source) Vref as is indicated by the dashed line. The collectorof T5 is coupled to the base of T6 and to a terminal 328. The emitter ofT6 is coupled to the anode of GDSC, to the cathode of D1, and to aterminal 330. The cathode of GDSC is coupled to a second terminal of CL1and to terminal 332. The anode of D1 is coupled to the emitter of T8 andto a terminal 338. The base of T8 is coupled to the emitter of T10 andto a terminal 340. The collectors of T8 and T10 are coupled together tothe emitter of T7 and to a terminal 342. The base of T7 is coupled to aterminal 344 and to the emitter of T9. The collectors of T7 and T9 arecoupled to one terminal of R4 and to a terminal 346. A second terminalof R4 is coupled to a first terminal of R3 and C1 and to a terminal 348.A second terminal of C1 is coupled to a terminal 352 which is coupled toa potential supply (source) Vref. A second terminal of R3 is coupled toa terminal 350 which is coupled to a potential supply (source) V2. Thebase regions of T9 and T10 are photosensitive. Terminal XO is coupledthrough a resistor R5 to a terminal 354 which is coupled to a potentialsupply (source) V3. Terminal YO is coupled through a resistor R6 to aterminal 356 which is coupled to a potential supply (source) V4.

The combination of T7 and T9 form one photo-Darlington pair and thecombination of T8 and T10 form a second photo-Darlington pair. These twophoto-Darlington pairs are connected together in series. With lightincident on the photosensitive base regions 341, 345 of T9 and T10,terminals 346 and 338 are coupled together via a relatively lowimpedance path. With the light removed, the two terminals areelectrically isolated. The series combination of two suchphoto-Darlington pairs is used to provide a high voltage and highcurrent capability switch. This allows control circuitry 310 to beelectrically isolated from V2 by eliminating incident light on the basesof T9 and T10. Other high voltage and high current switches can besubstituted for the photo-Darlington pairs.

The basic operation is as follows: Assuming XO and YO are coupledthrough current limiting resistance (not illustrated) to +220 volts and-220 volts, respectively, conduction occurs through GDSL2 if thepotential of gate terminal 332 is at a potential level which is near orbelow +220 volts. With V1=+320 volts, V2=+285 volts Vref=0 volts, andV3=V4=-48 volts, and current limiter CL1 limiting current therethroughto 1-14 microamperes, control circuitry 310 is capable of controllingthe state of GDSL1 and GDSL2 by providing the needed potentials atterminal 332 and a source of current into terminal 332.

Assuming first that it is desired to set GDSL2 to an ON (conducting)state, an input voltage signal having a level of typically +315 volts (alow or "0") is applied to input terminal 312 and light is illuminatedonto the photosensitive bases of T9 and T10. The emitter-base junctionsof T2, T3, and T4 become forward-biased and the potential of the base ofT1 (terminal 320) reaches a sufficiently positive potential with respectto the emitter (terminal XO) to forward-bias the emitter-base junctionof T1 and thereby cause T1 to turn ON and pull down the potential of thecollector thereof (terminal 332) to a value close to that of thepotential of XO. This leaves the gate and anode of GDSL2 at close to thesame potential and thus GDSL2 is in the ON sate and conducts currentfrom XO to YO. With T3 biased ON and conducting T4 is biased on sincethe emitter-base junction thereof is forward-biased. Thus an electricalpath through T4, CL1, T1 and R5 exists between V1 and V3. With T4 biasedON and conducting current through the emitter-collector thereof, thepotential appearing at the base of T5 (terminal 326) is insufficient toforward-bias the emitter-base junction of T5 since the collector-emitterpotential of T4 is designed to be less than the potential needed toforward-bias the emitter-base junction of T5. Thus, T5 is biased OFF andterminal 328 is electrically isolted from V1. Because the base terminal328 of T6 is electrically floating, T6 is biased off. Terminals 328 and330 are now electrically isolated from V1. The gate (terminal 322) ofGDSC is at a potential near V1 since T3 is biased on. The anode terminal330 is at a potential between approximately V1 and V2. While the anodeterminal 330 is near V1 in potential, GDSC is ON and conducts until thepotential of anode terminal 330 drops to approximately 20 volts belowthe potential of gate terminal 322. GDSC then switches OFF andconduction therethrough ceases. In addition, terminal 332 is alsoisolated from V2 since GDSC is OFF.

It is thus clear that branch circuit 310A serves to maintain load switchGDSL2 in the ON state and thus allows conduction therethrough.

Assume that it is now desired to switch GDSL2 to the OFF (blocking)state. Input terminal 312 is set to the level of +320 volts (a highlevel or "1"). This turns off T2, T3, and T4. T5 now becomes biased onand is typically operated in saturation such that terminal 328 rises inpotential and forward-biases the emitter-base junction of T6. Thisbrings the voltage of 330 near V1 and thus switches GDSC to the ONstate. This causes terminal 332 to rise in potential to a level close toV1. This potential on terminal 332 is sufficient to switch GDSL2 to theOFF (blocking) state if there is a sufficient positive current flow intothe gate of GDSL2. Minority carriers (e.g., electrons) emitted at thecathode of GDSL2 and collected at the gate (terminal 332) constitute theequivalent of positive current flow from V1 through T5, the emitter-basejunction of T6, GDSC, and into the gate of GDSL2. This current flow canbe substantial and as a result it is necessary to have a high voltageand current device such as GDSC to switch GDSL2 to the OFF state. A highvoltage and high current transistor would be expensive.

T5 is typically designed to have a relatively low current handlingcapability. As the current flow through T5 begins to increase, thevoltage drop across the collector-emitter of T5 increases significantlyuntil the potential of terminal 328 decreases to a level near V2. T5then essentially limits further conduction therethrough. D1, which hadpreviously been reverse-biased, now becomes forward-biased. With lightincident on the photosensitive bases 345 and 341 of T9 and T10,respectively, positive current flows from C1 and V2 into terminal 330and through GDSC and into the gate of GDSL2. The potential of terminal330 drops to a level below but close to that of V2. The values of R3,R4, C1, and the potential of V2 are selected to provide substantiallymore current than can be provided by T5. Accordingly, GDSL2 is switchedto the OFF state.

It is thus clear that 310A serves to essentially maintain GDSL2 in theOFF state and 310B serves to switch GDSL2 to the OFF state and totherefore help interrupt or inhibit conduction between XO and YOindependent of the potentials applied to XO and YO so long as theseapplied potentials are within preselected limits.

If GDSL2 fails to switch OFF C1 becomes essentially discharged. Then thelight incident on T9 and T10 is removed allowing C1 to recharge andterminal 312 is returned to +315 volts. This resets GDSC to the OFFstate and allows GDSL2 to continue to be in the ON state and conducting.Light is again illuminated on the photosensitive bases of T9 and T10 andthen the potential of 312 is raised back to +320 volts. GDSC is againswitched to the ON state and another attempt is made at causing GDSL2 tobe switched from the ON to the OFF state.

In one embodiment R1, R2, R3, R4, R5, and R6 are typically 1000, 10⁵,10⁴, 500, 10⁶, and 10⁶ ohms, respectively, and C1 is 0.1 microfarads.

The combination of T7, T9, T8, and T10, which act as essentially asingle high voltage and high current switch, are relatively expensivecomponents. R3, R4, C1, T7, T9, T8, and T10 can be shared between anumber of control circuitries 310.

If YO is more positive in potential than XO, then GDSL2 does not conductand GDSL1 is affected in essentially the same manner as is describedabove for the operation of GDSL2.

Referring now to FIG. 4, there is illustrated a switching system 400comprising control circuitry 410 (contained in the large dashed linerectangle), two pairs of bidirectional switches GDSL10, GDSL20, andGDSL3, GDSL4, and resistors R50, R60, R7, and R8.

Control circuitry 410 essentially comprises a first voltage branch 410A(illustrated within another dashed line rectangle) and a second voltagebranch circuit 410B (illustrated within still another dashed linerectangle). The components of 410B are essentially the same as thecomponents of 310B of FIG. 3, and they function in essentially the sameway. The components of 410A are essentially the same as those of 310A ofFIG. 1, except that four additional devices, n-p-n transistor T10A,p-n-p transistor T20A, and diodes D2 and D3 have been added tofacilitate the controlling of a second bidirectional switch whichcomprises gated diode switches GDSL3 and GDSL4. An output terminal 438of 410B is illustrated connected to the anodes of diodes D4, D5 . . .Dn. Each of these diodes represents a branch circuit which isessentially identical to 410A, and which has a separate pair ofbidirectional switches, like GDSL10, GDSL20, and GDSL3 and GDSL4 coupledthereto. The control voltage branch 410B of control circuitry 400 isthus shared by n other control circuits which are essentially identicalto 400.

D2 and D3 serve to electrically isolate terminals X00 and Y00 from X1and Y1. T20A serves to control T10A in the same manner that T20 controlsT10. Components of 400 which are essentially identical to components ofcontrol circuitry 310 of FIG. 3 have the same reference denotation witha "0" added thereto. Corresponding terminals have a "4" as the firstreference number whereas in FIG. 3 the first reference number is a "3".

Control circuitry 400 has been built and found to be functional. Fourvoltage branch circuits 410A and one shared branch circuit 410B wereused to control the states of eight pairs of gated diode switches. Eachof the pairs of gated diode switches were similar to structure 10 ofFIG. 1 except that there was no electrode coupled to region 40.Transistors T10 and T10A, and the corresponding six other transistors(not illustrated), were separate transistors. All of the circuitry ofthe first voltage branches 410A and the eight pairs of gated diodeswitches were fabricated on a single integrated circuit chip having anarea of 14 square millimeters using dielectric isolation of components.The structure of the junction transistors used were similar to thosedisclosed in U.S. patent application, Ser. No. 971,632, filed on Dec.20, 1978, which issued Nov. 4, 1980, as U.S. Pat. No. 4,232,328. Theaverage turn-on time of a gated diode switch used with 400 is 300microseconds. The turn-off time is 50 microseconds. The average powerdissipation of each of the branch circuits 410A is 5 milliwatts.

The embodiments described herein are intended to be illustrative of thegeneral principles of the present invention. Various modifications arepossible consistent with the spirit of the invention. For example,transistor T6 of FIG. 3 and T60 of FIG. 4 can be eliminated. In suchcases, the collector of T5 of FIG. 3 is connected directly to the anodeof GDSC and the collector of T50 of FIG. 4 is connected directly to theanode of GDSCO. Still further, other high voltage and high currentswitches, such as a gated diode switch, could be substituted for thephoto-Darlington pairs provided appropriate control circuitry isprovided to control same. Still further, the junction transistors couldbe replaced by field effect transistors provided the polarities andmagnitudes of the power supplies and circuit configurations areappropriately modified as is well known in the art. Still further, R1,R2, R3, R4, R5, and R6 can be standard integrated circuit resistors orpinch resistors. Still further, the current limiter can be a variety ofdifferent types of resistors or a junction field effect transistor.Still further, the configurations of T3, T4, and T5 of FIG. 3 and T30,T40, and T50 of FIG. 4 could be replaced by the circuitry illustratedwithin the dashed line rectanges A and A0 of FIGS. 1 and 2 of U.S.patent application, Ser. No. 972,024, now abandoned. Still further, ifat least one of the load GDSs of each pair is like GDS1 of FIG. 1, thenthe transistor which selectively causes the gate and cathode (anode) tobe close to each other in potential, transistor T1 (T10A, T10 . . . ),is part of the GDS and terminal 320 (420, 420a . . . ) is connecteddirectly to electrode 50 (or its equivalent) of the load GDS. Stillfurther, CL1 and CL10 can be connected to V2 and V20, respectively,instead of being connected to the cathodes of the respective GDSCs.Still further, D2 and D3 can be eliminated if a second control gateddiode switch, like GDSCO, is added with its anode common to the anode ofGDSC and its gate common to the gate of GDSC, and its cathode connecteddirectly to the gates of GDSL3 and GDSL4. In this case the cathode ofGDSCO is connected directly to the gates of GDSL10 and GDSL20. Stillfurther, T5 and T50 can be designed to have relatively high currentcapabilities. The load switches GDSLs can be switched to the OFF statesolely by the first branch circuits 310A and 410A so long as the currentflow through the load switches GDSLs is within preselected limits. Ifthis current flow is greater than these limits then the second branchcircuits 310B and 410B are necessary to switch the load switches GDSLsto the OFF state.

We claim:
 1. Circuitry for use with a first switching device (GDSL1) ofthe type comprising a semiconductor body (16) a bulk portion of which isof a relatively high resistivity, a first region (18) of a firstconductivity type and of a relatively low resistivity, a second region(24) of a second conductivity type opposite that of the firstconductivity type, the first (18) and second (24) regions beingconnected to output terminals of the switching device, a gate region(20) of the second conductivity type, the gate region (20) being coupledto a control terminal of the switching device (GDSL1), the first (18),second (24) and gate (20) regions being mutually separated by portionsof the semiconductor body bulk portion (16), the parameters of thedevice being such that, with a first voltage applied to the controlterminal of the first switching device (GDSL1), a potential isestablished within a cross-sectional portion of the bulk of thesemiconductor body (16) which is substantially different from that ofthe potential of the first (18) and second (24) regions and whichprevents current flow between the first (18) and second (24) regions,and that, with a second voltage applied to the control terminal of thefirst switching device (GDSL1) and with appropriate voltages applied tothe output terminals (XO, YO) of the first switching device, arelatively low resistance current path is established between the first(18) and second (24) regions by dual carrier injection, a secondswitching device (GDSC) of the same type as said first switching device,an output terminal 332 of the second switching device (GDSC) beingcoupled to the control terminal of the first switching device (GDSL1), afirst branch circuit (310A) coupled to the second switching device(GDSC) for controlling conduction between the first and second regionsthereof, characterized bycircuit means (T1) being coupled to the controlterminal (332) and to one of the output terminals (XO) of the firstswitching device (GDSL1) and having a control terminal (320) coupled toan input terminal (312) of the circuitry, the circuit means (T1) beingadapted to selectively cause the control terminal (332) of the firstswitching device (GDSL1) to be close in potential to that of the outputterminal (XO) of the first switching device (GDSL1) which is coupledthereto.
 2. The circuitry of claim 1 further characterized by:a secondbranch circuit (310B) coupled to the second switching device (GDSC); thefirst branch circuit (310A) being adapted to be coupled to a firstpotential source (V1); and the second branch circuit (310B) beingadapted to be coupled to a second potential source (V2) which is adaptedto supply current into the first output terminal of the second switchingdevice (GDSC) if said output terminal assumes a preselected potentiallevel.
 3. The circuitry of claim 2 further characterized by:the firstbranch circuit (310A) comprises a third switching device (T5) havingoutput circuitry (328) coupled to the first output terminal (330) of thesecond switching device (GDSC) and having a control terminal (326)coupled to an input terminal (312) of the circuitry, the third switchingdevice (T5) being adapted to selectively electrically isolate the firstoutput terminal (330) of the second switching device (GDSC) from thefirst potential source (V1); and the second branch circuit (310B)comprises a fourth switching device (T7, T8, T9, T10) having outputcircuitry (338) coupled to the first output terminal (330) of the secondswitching device (GDSC) and having a control portion (341, 345), thefourth switching device (T7, T8, T9, T10) being adapted to selectivelyelectrically isolate the first output terminal (330) of the secondswitching device (GDSC) from the second potential source (V2).
 4. Thecircuitry of claim 3 further characterized by a first resistor (CL1)which is coupled to the control terminal (326) of the third switchingdevice (T5).
 5. The circuitry of claim 4 further characterized in thatthe resistor is a current limiter (CL1) and that it is also coupled tothe second output terminal (332) of the second switching device (GDSC).6. The circuitry of claim 5 further characterized by a fifth switchingdevice (T3) which has output circuitry (318) adapted to be coupled tothe first potential source (V1) and to the control terminal (322) of thesecond switching device (GDSC), the fifth switching device (T3) having acontrol terminal (314) coupled to the input terminal (312) of thecircuitry.
 7. The circuitry of claim 6 further characterized by:thefirst branch circuit (310A) comprises a diode (D1) whose cathode iscoupled to the first output terminal (330) of the second switchingdevice (GDSC) and whose anode is coupled to an output terminal (338) ofthe fourth switching device (T7, T8, T9, T10); sixth (T2), seventh (T4),and eighth (T6) switching devices which each have a control terminal andoutput circuitry; the control terminal (314) of the sixth switchingdevice (T2) being coupled to the circuitry input terminal (312) and theoutput circuitry being coupled to the output circuitry (318) of thefifth switching device (T3), the control terminal of the seventhswitching device (T4), and to the control terminal (320) of the circuitmeans (T1), a capacitor (C1); second (R1), third (R2), fourth (R3), andfifth (R4) resistors; the second resistor (R1) being coupled to thecircuitry input terminal (312) and to the control terminal (314) of thesixth switching device (T2); the third resistor (R2) being coupled tothe output circuitry (316) of the sixth switching device (T2) and to theoutput circuitry (318) of the fifth switching device (T3); the fifthresistor (R4) being coupled to an output terminal (346) of the fourthswitching device (T7, T8, T9, T10) and to a first terminal (348) of thecapacitor (C1) and to the fourth resistor (R3); and the fourth resistor(R3) being adapted to be coupled to the second potential source (V2). 8.The circuitry of claim 7 characterized by:the third (T5), fifth (T3) andsixth (T2) and seventh (T4) switching devices are p-n-p transistors; thecircuit means (T1) is an n-p-n transistor; the fourth switching device(T7, T8, T9, T10) comprises a first Darlington pair of transistors (T7and T9), and a second Darlington pair of transistors (T8 and T10), withtwo of the transistors (T9 and T10) having photosensitive base regions(345, 341).
 9. The circuitry of claim 8 characterized by:sixth (R5) andseventh (R6) resistors; the sixth (R5) resistor being coupled to one ofthe output terminals (XO) of the first switching device (GDSL1) and theseventh (R6) resistor being coupled to the other output terminal (YO);an eighth switching device (T6) which is an n-p-n transistor with thecollector (322) coupled to the output circuitry of the fifth switchingdevice (T3), the base coupled to the output circuitry (328) of the thirdswitching device (T5), and the emitter coupled to the first outputterminal (330) of the second switching device (GDSC).
 10. The circuitryof claim 9 characterized by:the first (GDSL1) and second (GDSC)switching devices being gated diode switches.
 11. The circuitry of claim1 characterized by:the circuit means (T1) being an integral part of thefirst switching device (GDSL1); and the first switching device (GDSL1)having a shield (base) region (22, 40, 46) which surrounds the secondregion (24), said shield (base) region (22, 40, 46) being of the firstconductivity type and having a separate control terminal (50) connectedthereto.